Do a search for "homework" and you should find plenty of examples of systems that require that sort of input and output. Rememeber when counting IO that motors normally have at least 3 points associated with them - 2DI, 1DO (I1: contator closed feedback, I2: thermal overload healthy, O1: close contactor. Optionally, DI3: motor maintenance switch feedback). So you could easily make a conveyor system with 3 motors and some sensors that does something useful, for instance sorting packages by weight into different lanes, or a level and filtering application for a swimming pool, or anything you like really. Or a consider what might be needed to implement a controller for something like, say, a wind turbine. Think of almost any automated practical task that does not require very high speed control, light weight, enormous project size or some other specialty characteristic and a PLC can be applied usefully to the problem.
